1. Field of the Invention
This invention relates to an apparatus for driving a light-emitting display constructed of light-emitting elements such as organic EL elements and light-emitting diodes.
2. Description of the Related Art
Where an image is displayed on the light-emitting display, each light-emitting element must be illuminated with the luminance corresponding to the luminance value of each of pixels of an image signal.
The technique for illuminating each light-emitting element with the luminance corresponding to the luminance value of the pixel includes an analog method and a time-divisional method.
The analog method is to vary the driving current of illuminating the light-emitting element according to the luminance value. The time-divisional method is to turn on/off the driving current, which is maintained constant, according to the luminance value, thereby varying its “ON” time.
The analog method requires linearity at high accuracy in order to vary the driving current according to the luminance value. Therefore, the analog method has a disadvantage that a driving section is up-sized and the value of the driving current varies according to a temperature.
On the other hand, in the time-divisional method in which requires a constant current to be produced, the driving section is down-sized and has a good temperature characteristic.
New referring to FIG. 11, an explanation will be given of a driving apparatus in a time-divisional system.
In FIG. 11 reference numeral 50 denotes a frame memory for storing an image signal (pixel data) corresponding to its one frame; 51 a pattern memory; 52 a read section; 53 a driving section for producing a constant driving current; and 54 a light-emitting display.
For simplicity of explanation, the explanation will be made for a single pixel.
The read section 51 reads the pixel data stored in the frame memory in a frame period.
The luminance value of the pixel data is represented by a binary number of k (2k) Specifically, where k=8, the luminance is represented by 256 levels.
The pattern memory 51 stores schedule data (pattern information) for turning on the driving section 53 for the pixel data read by the read section 50.
FIG. 12 shows a concrete example of the contents of the pattern memory 51 where k=3.
The addresses of the pattern memory 51 are correlated with the pixel data read from the frame memory 50. The pattern information is recorded as the bit information of 2k−1.
Namely, where k=3, the address is represented by 3 bits and the pattern information is represented by 7 bits.
For example, as seen from FIG. 12, where the address is ‘000’, the pattern information is ‘0000000’, and where the address is ‘001’, the pattern information is ‘1000000’. Likewise, the pattern information will be previously stored as seen from FIG. 12.
Using the addresses of the pixel data read from the frame memory 50, the read section 52 reads the pattern information stored in the pattern memory 51 and sequentially sends it to the driving unit 53 in a period of 1/(2k−1) of the frame period.
Where the signal sent from the read section 52 is “1”, the driving section 53 supplies a constant current to the light-emitting display 54, whereas the signal sent from the read section 52 is “0”, the driving section 53 stops supply of the current.
Generally, the image data has the luminance values of 8 bits or larger, i.e. 2k−1=255 level or larger. In this case, the pattern memory requires 256 addresses and memory capacity of 255 bits.
In the conventional apparatus for driving a light-emitting display in a time-divisional manner, where the image signal is represented by a binary number of k, the driving section 53 is on/off controlled in a period of 1/(2k−1) of the frame period, thereby requiring a high speed operation.